TBH the Att/Def multipliers are not necessary once you build up your team... Had to go without this mod when it became VIP only and my setup worked fine without the need for any multipliers.
For those who are still facing black screen:
After doing all the steps in OP, reinstall the apk without uninstalling the game (I. E install as an update over the apk.
I had the same problem until that update fixed it.